Voltage-controlled oscillators that supply a signal of a frequency that accords with the input voltage are used in, for example, PLL circuits (phase-locked-loop circuits) or frequency synthesizers that implement high-speed transition (hopping) of frequency and that feature compact size and low spurious power.
FIG. 1 is a block diagram showing the configuration of a voltage-controlled oscillator of the related art. In FIG. 1, the voltage-controlled oscillator includes input terminal 1, voltage/current converters 2 and 3, current-controlled oscillator 4, and output terminal 5.
Voltage/current converters 2 and 3 convert the input voltage that is applied to input terminal 1 to a current that accords with this input voltage. Current-controlled oscillator 4 generates a signal of a frequency that accords with the sum of the currents that were converted in each of voltage/current converters 2 and 3.
Voltage/current converters 2 and 3 are each subjected to control such that the input/output characteristic of the voltage-controlled oscillator is linear. For example, when the conversion gain of voltage/current converter 2 is reduced, voltage/current converter 3 is subjected to control such that its own conversion gain increases.
In this way, the frequency range (hereinbelow referred to as the “variable frequency range”) in which the input/output characteristic of the voltage-controlled oscillator is linear can be broadened.
In a voltage-controlled oscillator of this type, however, there is one input/output characteristic, and phase noise therefore increases. This problem occurs because, in order for a voltage-controlled oscillator having a single input/output characteristic to obtain the same variable frequency range as a voltage-controlled oscillator having a plurality of input/output characteristics, the conversion gain of the voltage-controlled oscillator must be increased.
Patent Document 1 (JP-A-2003-69390) discloses a PLL circuit that is provided with voltage-controlled oscillator in which the input/output characteristic is linear in the variable range of the input voltage. Normally, the input/output characteristic is not linear when the input voltage approaches the power-supply voltage that controls the operation of the voltage/current converter. In the voltage-controlled oscillator described in Patent Document 1, the input/output characteristic can be made linear and the variable frequency range can be broadened even when this input voltage approaches the power-supply voltage.
More specifically, in this voltage-controlled oscillator, the input voltage is converted to current in a voltage/current conversion circuit that operates at a second power-supply voltage that has a high voltage value. This current is supplied by way of a first and second current-mirror circuit that operate at the second power-supply voltage to a third current-mirror circuit that operates at a first power-supply voltage that is lower than the second power-supply voltage and a plurality of first transistor gates for controlling current to a ring oscillator.
In this way, the variable frequency range can be broadened even when increase in the conversion gain of the voltage-controlled oscillator is not increased.
Patent Document 1: JP-A-2003-69390